. The following experimental studies are proposed in this application: 1. Study the molecular biology of mycoplasma fermentans (the incognitus strain) which has direct implication in clinical diagnosis, in understanding pathogenesis, and in future vaccine development. 2. To identify M. fermentans specific repetitive DNA sequences to be used to develop highly sensitive and specific PCR assays to detect M. fermentans related mycoplasma infection in patients with AIDS. 3. To develop serological assays detecting specific antibodies produced in patients with AIDS against surface antigens of M. fermentans. 4. To develop reliable and practical culture systems, to identify the best disease stage for direct isolation, and to characterize the new clinical isolates of M. fermentans related mycoplasmas from AIDS patients. 5. To correlate the results of antibody positivity, PCR evidence of the mycoplasma infection and culture isolation with disease stages of AIDS. To assess if the presence of mycoplasma infection plays any role in the disease of AIDS.